iMac G3 (slot loader) buzzing, what part?

I bought a second hand iMac G3 off eBay for a pretty good deal, and am trying to figure out where the buzzing is coming from.

It doesn't come with any CDs so I don't think I can do a hard wear test, but I've noticed it is happening even when the display is off, and is in sleep (although it gets less audible in sleep).

Another thing I noticed is that in OS X it will stop after a while of not using it, despite the screen being on and still showing the image, is this the hard drive stopping? When I go and touch a key or move the mouse it starts again.

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